1. Field of the Invention
The present invention relates to a method for producing a liquid crystal display device, in which a pretilt angle and a tilt direction upon application of a voltage of liquid crystal molecules are controlled by using a polymer.
2. Description of the Related Art
In a liquid crystal display device of a multi-domain vertical alignment mode (MVA-LCD), a liquid crystal having negative dielectric anisotropy is vertically aligned, and the alignment direction of the liquid crystal upon application of a voltage is controlled to certain directions in pixels by utilizing an alignment controlling structure, such as linear protrusions and slits provided on a substrate, without rubbing.
An MVA-LCD has such an advantage that it has excellent viewing angle characteristics in comparison to a liquid crystal display device of a TN mode, but it disadvantageously exhibits a low white luminance and low luminosity. The disadvantages are caused by such a mechanism that the area occupied by the alignment controlling structure forms segmentation boundaries in alignment of the liquid crystal, and the area is viewed with low luminosity to lower the light transmittance of the entire pixels. In order to remove the disadvantages, the interval of the alignment controlling structure is sufficiently broadened. In this case, however, a prolonged period of time is required for stabilizing the alignment of the liquid crystal due to the small amount of the alignment controlling structure for controlling the alignment, whereby the response time is prolonged.
In order to obtain an MVA-LCD having a high luminance and a short response time, it has been proposed that an alignment direction of a liquid crystal upon driving is controlled by using a polymer. In this method, a liquid crystal material obtained by mixing a liquid crystal and a monomer (polymerizable component) capable of being polymerized with heat or light is sealed between two substrates. The monomer is polymerized by irradiating the liquid crystal layer with UV light in a state where the liquid crystal molecules are tilted by applying a prescribed voltage between the substrates, so as to form a polymer. A liquid crystal layer controlled to a prescribed alignment direction and a prescribed pretilt angle is thus obtained even after removing the applied voltage, owing to the polymer formed in the vicinity of the surfaces of the substrates. Accordingly, a rubbing treatment of an alignment film can be omitted.
The aforementioned related art can be found, for example, in JP-A-7-5444, JP-A-2001-33767 and JP-A-5-158020.
However, it has been found in the liquid crystal display device produced by using the method of controlling the alignment direction of the liquid crystal molecules upon driving with a polymer that fluctuation in thickness of the cells largely influences the optical characteristics. Specifically, the cell thickness influences the optical characteristics in both cases upon polymerizing the polymer and upon practically driving the device, and therefore, such problems arise in that fluctuation in optical characteristics is increased in comparison to a device produced without polymerization, and luminance unevenness is liable to occur due to the luminance distribution of the UV light or the temperature distribution upon polymerizing the monomer.
The conventional liquid crystal display devices also have the following problems.
(1) The optical characteristics; such as the γ characteristics and the black luminance, are fluctuated upon changing the materials for the color filter layer and the alignment film, whereby the driving circuit is necessarily modified.
(2) A distribution in luminance occurs within the panel due to influences of wiring resistance, which is recognized as unevenness in luminance.
(3) The transmittance with respect to the birefringence of the liquid crystal is different by the colors of the color filter, so as to cause coloration in halftone.
A liquid crystal display device is demanded to have bright display without unevenness, high speed response, less color change in half tone, and constant optical characteristics, such as the γ characteristics and the black luminance, from the standpoint of mass production.